The present invention relates to a method for soldering hard substances onto a base or substrate body formed of steel at temperatures above 570.degree. C. using a three-layer solder. In carrying out the invention, the middle layer of the three-layer solder consists of a copper alloy or nickel alloy which is provided on both sides thereof with a layer of a hard solder whose working temperature is located at least 50.degree. C. below the melting point of the copper alloy or nickel alloy of the middle layer.
Hard substances, especially cemented carbide and cermets are extensively used in cutting technology for fabrication of work tools used in working operations requiring cutting of wood, steel and stone. It is necessary for these purposes to connect the hard-substance parts to a base body of steel. This takes place as a rule by means of clamping connections or screw connections or, more preferably, by soldering.
When these working tools are used, high and complex stresses such as tensions of pressure, traction and shearing occur in the connection zone between the hard-metal part and base body. In addition, during soldering significant stresses arise during the cooling down period due to the different coefficients of thermal expansion. Such stresses can be compensated for by using solders which remain ductile. Ductile solders are known in the art such as, for example, CuMnNi alloys, CuMnCo alloys, CuMnSi alloys and AgCuZnMnNi alloys, which exhibit mechanical strengths sufficiently high for many applications. For extreme applications like those which can frequently occur in cutting technology, solders with greater strengths are required.
German OLS 40 36 777 teaches the use of precipitation-hardenable solders such as CuNiSi alloys, CuBe alloys, CuZr alloys or CuTi alloys, for connecting cemented carbide parts to base bodies or substances of steel. These solders exhibit a high ductility after cooling down and achieve a high mechanical strength after precipitation-hardening by means of a tempering treatment by heating.
However, a disadvantage of this method is the fact that on account of the high melting point of these precipitation-hardenable copper alloys high thermal stresses arise in the composite of hard metal and steel. This can result in failure of the structural part, especially in the case of hard metals with a low percentage of cobalt binder.
The term "precipitation hardening" as used herein is well understood in the art. Reference is made to The Encyclopedia of Chemical Technology by Kirk-Othomer, 3rd Edition, Volume 12, page 425 and Volume 15, page 335; incorporated herein by reference.
Multilayer solders are known in soldering technology. They are used, for example, to manufacture brittle, non-deformable solder parts by plating the ductile solder components onto each other and producing corresponding solder form parts. The actual solder alloy is not produced until during the melting of the laminated solder.
EP patent 677,012 teaches a three-layer solder consisting of an intermediate layer of steel, copper or copper-nickel alloy on which a solder layer is plated on both sides. The intermediate layer remains fixed during the soldering process and has the task of strengthening the solder connection in order to receive thermal tensions. This solder is also used for hard metal tools but still does not exhibit optimum strength values.
Multilayer solders are also described in U.S. Pat. No. 3,676,088 in which the intermediate layer consists of a copper alloy. These solders function as replacements for the brittle copper-phosphorus solders and are not used to solder hard metals onto steel.
A multilayer solder according to German patent 863 737 is known for soldering hard metals onto steel which solder consists of an intermediate layer of copper or nickel plated on both sides with a silver solder. The soldering is followed by a heat treatment in order to achieve a further diffusion between the silver and the metal of the intermediate layer. High hardness values can therefore not be achieved.